text-font should support an "outline" argument

[schanzer]

Loosely paraphrased from @shriram : "It's 2015, and we're still designing languages that can't support meme generation natively?"

Proposal: text-font should support a 9th argument (boolean), that causes the text to be rendered as white with a colored stroke. I considered overloading the "style" argument, but that seems like an uglier kludge. This change has been implemented in WeScheme (demo at http://192.wescheme.appspot.com/openEditor?publicId=fzUP5QlHU0).